This invention pertains to digital printing and more particularly to the metering and auditing of the dots or drops produced by a digital printer.
Printers that print characters in the form of dots have been utilized in postage meters and other devices. The aforementioned printers form characters and/or graphics from a matrix of dots. Unlike the fully formed character printing methods, the printing elements are organized in rows or columns which print dots. A character in a dot printer is formed sequentially by printing at one time all the selected dots, respectively, in a column or a row. Graphics are made possible by precisely positioning dots on a page.
Printers that print characters and graphics by depositing drops of ink on a medium have been utilized in postage meters and other devices. The aforementioned printers form characters and graphics by selectively firing droplets of ink onto a surface. The ink dries upon its absorption into the substance.
Laser printers print characters and graphics by utilizing a focused laser beam and a rotating mirror to draw an image of the desired page on a photosensitive drum. The laser is pulsed periodically or fired periodically to produce small discharged areas on the photosensitive drum that represent the image. The charged image attracts and holds toner. A piece of paper is rolled against the drum while a charged plate behind the paper attracts the toner away from the drum and onto the paper. Heat and/or pressure is then applied to fuse the toner to the paper.
Dot matrix printers print characters. A dot matrix printer may have a 9 or 24 pin head. The pins impact the paper through a ribbon, creating patterns of dots in the shape of letters and numbers in multiple fonts and type sizes.
Thermal matrix printers have an array of 100 to 200 pins which are placed in contact with thermally sensitive paper. The pins are pulsed or fired with electrical current heating the pins. The heat produced darkens selective areas of the moving paper.
Printers that print by using dots and drops are commercially available as desk top printers and are often utilized as output devices of personal computers. The wide use of the above printers has made it easier to forge documents. Thus, additional security is needed to determine the authenticity of the printed document. One method that has been proposed for providing security is to print encrypted information in the document and decrypting the information at a later time to authenticate the document. One of the disadvantages of the foregoing is that it may be necessary to use a large amount of space on the document to prevent the encrypted information from being decrypted.
Another method that has been proposed for providing security to documents is to print authenticating text in invisible ink on the document as a means of authenticating the document. A luminescent ink may also be used for similar security purposes. One of the disadvantages of the foregoing is that it may be necessary to use special chemicals or an ultraviolet light source to read the authenticating text.
Another method utilized by the prior art for providing security to documents involved the hiding of some information in the document or the modification of some information in the document. The hidden or modified information may be placed in graphics contained in the document. The hidden or modified information was accurately placed so as not to disturb the information. One of the disadvantages of the above is that it is difficult to read the hidden or modified information.
This invention overcomes the disadvantages of the prior art by providing a system that makes it more difficult to print fraudulent documents. The apparatus of this invention provides a device for verifiable security in a postage meter or other devices i.e., tax meter, stock certificate printer, bank note printer, certified check printer, etc. using dot or drop printing. Security is achieved by counting the number of signal pulse firings that are used to produce ink drops or ink dots that are required to produce the entire document or specific regions of the document. The aforementioned may be accomplished by storing the printer firings in a two dimensional, non-volatile memory array and auditing the printed material on the mail piece by using the firings of the printer to compare the value that is printed on the mail piece to the value decremented in the registers of the meter. The apparatus of the invention determines what is printed on the mail piece by reading the printer firings (that represent the alphanumeric characters or other data produced) and comparing it with the value stored in non-volatile memory.
An advantage of this invention is that it can accurately determine the amount of postage that is printed without encountering errors due to paper, velocity of the paper, ink, paper skew, etc.